3 stand out for me, i will include reasons.
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Stalingrad : Losing the entire 6th army was a demoralizing blow and one of the most important turning points of the war. It wasnt over yet, but set the tone for the remainder of the war.
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Kursk : The last hurrah, basically fought to a stalemate which meant Russian victory. There were no replacements available for the masses of equipment and manpower lost here by the Germans. The retreat all the way to Berlin began here. This battle was Russias D-day.
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Ardennes, the Battle of the Bulge: Hitlers plan to force the allies back into the sea. If it had been successful and peace made with the Brits and Uncle Sam more troops would have been available for the eastern front and the German industrial belt may have been able to keep turning out new Tiger tanks. Of course at this point it may have already been too late but who knows.